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Abstract:

An optical space observation system comprises a primary mirror, a
secondary mirror, a supporting base containing the primary mirror, on
which a mechanical structure bearing a support of the secondary mirror is
positioned, and an optical measurement means, said mechanical structure
comprising a plurality of mechanical arms. The system also has a system
comprising a plurality of actuators positioned on the supporting base,
said actuators being connected to the lower ends of said mechanical arms
and the upper ends of said mechanical arms being connected to the support
of the secondary mirror on the periphery of the support. A space
telescope is intended to be loaded in a launcher and put into orbit. The
invention is intended in particular for a telescope having a deployable
secondary mirror and means for active control of the optics.

Claims:

1. An optical space observation system comprising: at least a primary
mirror, a secondary mirror, a supporting base containing the primary
mirror, on which a mechanical structure bearing a support of the
secondary mirror is positioned, and an optoelectronic measurement means,
said mechanical structure comprising a plurality of mechanical arms and
the optoelectronic measurement means capturing images acquired by the
optical space observation system, a calculation means for calculating
data for correcting the positioning of the secondary mirror on the basis
of data delivered by the optoelectronic measurement means, a plurality of
actuators positioned on the supporting base, said actuators being
connected to the lower ends of said mechanical arms and the upper ends of
said mechanical arms being connected to the support of the secondary
mirror on the periphery of the support, wherein the positioning of the
secondary mirror is adjusted by means of actuators displacing the lower
ends of said mechanical arms on a translation path as a function of the
correction data, the optical measurement means, the calculation means and
the system of actuators constituting an active control chain for
correcting the positioning of the secondary mirror in order to adjust the
observation configuration of the optical system.

2. The system according to claim 1, wherein the length of the mechanical
arms is constant during the correction phase.

3. The system according to claim 2, wherein the calculation means
compiles the positioning corrections of the secondary mirror by means of
a wavefront reconstruction algorithm.

4. The system according to claim 3, wherein the system of actuators and
the mechanical structure bearing the support of the secondary mirror
constitute mechanical means intended to introduce defects on the measured
images.

5. The system according to claim 1, wherein the mechanical structure
bearing the support of the secondary mirror is a deployable structure
such that in a first configuration the support of the mirror rests
directly on the supporting base and, in a second configuration, the
support of the mirror is in a position separated from the supporting
base.

6. The system according to claim 5, wherein the system is installed
inside a spacecraft and the mechanical structure is configured in said
first configuration when the system is installed in said spacecraft, and
in said second configuration when the system is in observation mode.

7. The system according to claim 1, wherein the system of actuators has
actuators with translation axes perpendicular to the upper plane of the
supporting base.

8. The system according to claim 7, wherein the system of actuators has
six actuators distributed over the periphery of the supporting base in
order to displace the support of the secondary mirror in six degrees of
freedom.

9. The system according to claim 8, wherein the mechanical structure
bearing the support of the mirror has six mechanical arms.

10. The system according to claim 9, wherein the secondary mirror is
immobile on the support.

Description:

[0001] The field of the invention relates to optical systems for space
observation and, more particularly, to optical observation systems
intended to be loaded on board spacecraft and be deployed in space.

[0002] Here, optical systems for space observation are intended to mean
telescopes making it possible to obtain high-resolution images for
observation of the Earth from space or for deep space observation. These
telescopes are for example of the Cassegrain, Gregorian, Korsch,
Ritchey-Chretien or Newtonian type, etc. The optical detectors of a space
telescope must be capable of recording images of objects which have very
low luminosity, generally requiring exposure times under the limits
imposed by the stabilization capacity of the optical system. These
applications require the use of space observation systems with ever
greater size and higher performance.

[0003] Space telescopes are intended to be fitted to satellites intended
to be put in orbit. Currently, telescopes are designed to be able to
undergo the vibrations of the take-off phase for sending the satellite
into space, then the thermoelastic orbital stresses, without significant
modifications of the optical adjustments carried out on the ground. These
constraints are leading to the design of extremely robust structures
employing exotic materials, hyperstable connections and ultraprecise
temperature control systems. The consequences of such a design of the
structures are high weight and cost.

[0004] Furthermore, in the near future, the collecting surface
requirements of future observation systems mean that their structures
will tend to be deployable. The telescopes mentioned above comprise a
primary mirror, a secondary mirror separated from the primary mirror by a
selected distance and a supporting base containing the primary mirror, on
which a mechanical structure bearing a support of the secondary mirror is
positioned. A deployable structure is intended to mean a structure in
which the secondary mirror can, in a first configuration, be in a
position close to or in contact with the supporting base and, in a second
configuration, can be separated from the primary mirror. By virtue of
this type of structure, it is possible to reduce the bulk of a satellite
when transporting it from the ground to its mission orbit, and
consequently to load a larger number of satellites on board the
launchers. For these structures, however, a system allowing optical
adjustment to be carried out in flight must be envisaged.

[0005] To this end, there are telescopes having systems for mechanical
adjustment and correction consisting of a hexapod at the interface of the
secondary mirror. This hexapod comprises the system of actuators, the
control system of the actuators and the supply cabling. Such a solution
involves the latter elements also being deployable. For example, the
American patent U.S. Pat. No. 6,477,912 is known which describes a
mechanical system for controlling a plate which, for example, may be a
telescope secondary mirror.

[0006] Active control systems for correction often use wavefront
reconstruction algorithms. For example, the American patent U.S. Pat. No.
4,309,602 may be mentioned which describes a control solution for an
optical system using, for example, an algorithm for wavefront
reconstruction by phase diversity.

[0007]FIG. 1 represents a simplified diagram of an existing space
telescope, having a primary mirror and a secondary mirror 106. The
primary mirror (not shown) is positioned on a supporting base 100, and
the secondary mirror 106 is supported by a first mechanical structure 110
which is non-deployable and immobile, comprising three pairs of
mechanical arms 101 and intended to separate the secondary mirror from
the primary mirror, and by a second mechanical structure consisting of
mechanical elements 103 connecting the secondary mirror 106 to a plate
107. The position of the secondary mirror is modified by the actuators
102 by displacing the plate 107. The plate 107, the secondary mirror 106,
the actuators 102 and the second mechanical structure form the hexapod
structure 120 allowing the position of the secondary mirror to be
modified. This hexapod also includes the electronic systems for supply,
control, etc. This latter solution, although it does make it possible to
correct the position and the orientation of the secondary mirror, induces
an increase in mass at a position separated from the centre of gravity of
the optical system. This configuration reduces the agility of the system
and lowers the vibration frequencies of the first natural modes of the
telescope. Furthermore, this increase in mass at the secondary mirror
requires an increase in the rigidity of the structure so as to be able to
withstand the accelerations during the take-off phase, and consequently
an increase in the mass of the structure.

[0008] The French patent 2628670 in the name of INRIA (Institut National
de Recherche en Informatique et en Automatique) is known, describing an
articulated device for the field of robotics, in particular for the
design of a robot hand or alternatively for the design of a flight
simulator. This articulated device allows high positioning accuracy.

[0009] It is an object of the invention to overcome the drawbacks of the
solutions mentioned above and to provide an optical system having means
for active control of the optics of greater scope, presenting better
performance and withstanding the stresses of space use.

[0010] More precisely, the invention relates to an optical space
observation system comprising at least a primary mirror, a secondary
mirror, a supporting base containing the primary mirror, on which a
mechanical structure bearing a support of the secondary mirror is
positioned, and an optoelectronic measurement means, said mechanical
structure comprising a plurality of mechanical arms and the
optoelectronic measurement means capturing images acquired by the optical
space observation system. The optical space observation system according
to the invention is characterized: [0011] in that it has a calculation
means calculating data for correcting the positioning of the secondary
mirror on the basis of data delivered by the optoelectronic measurement
means, [0012] in that it also has a system comprising a plurality of
actuators positioned on the supporting base, said actuators being
connected to the lower ends of said mechanical arms and the upper ends of
said mechanical arms being connected to the support of the secondary
mirror on the periphery of the support, [0013] and in that the
positioning of the secondary mirror is adjusted by means of actuators
displacing the lower ends of said mechanical arms on a translation path
as a function of the correction data.

[0014] During the correction phase, the length of a mechanical arm is
constant and the secondary mirror is immobile on its support.

[0015] The invention is advantageous because the part dedicated to the
electronic and mechanical means of the active the control system for the
optics of the telescope is positioned on the supporting base. The mass is
thus principally distributed over the base and the centre of gravity is
therefore lowered.

[0016] In a first embodiment, the mechanical structure bearing the support
of the secondary mirror is a deployable structure such that in a first
configuration the support of the mirror rests directly on the supporting
base and, in a second configuration, the support of the mirror is in a
position separated from the supporting base.

[0017] Since the structure of the telescope no longer has the system of
actuators at the secondary mirror, the solution facilitates the use of a
telescope structure with a deployable architecture of the secondary
mirror, for the reason that the mechanical structure can be designed with
fewer dimensional stability constraints. This is because the adjustments
of the telescope are carried out in orbit. Advantageously, the optical
system also exhibits better agility because the structure bearing the
secondary mirror has less mass.

[0018] The dimensions of the space telescope can also be increased, thus
making it possible to design more highly performing optical systems.

[0019] Advantageously, the space telescope can be installed inside a
spacecraft and the mechanical structure is configured in said first
position when the system is installed in said spacecraft, and in said
second position when the system is in observation mode. The deployable
structure reduces the bulk of the optical system and consequently makes
it possible to transport a larger number of systems inside the launcher.

[0020] In a second embodiment, the mechanical structure is a
non-deployable architecture. Since the electronics and mechanics for
controlling the secondary mirror are positioned on the supporting base,
the upper part of the telescope for the support of the secondary mirror
can be adapted easily to the supporting base.

[0021] In both embodiments, the invention is advantageous because the
mechanical architecture makes the design and development of the optical
system more flexible than a solution with the active control system at
the secondary mirror. Specifically, the solution makes it possible to use
an architecture with a deployable or non-deployable secondary mirror. The
supporting base constitutes a standardized mechanical base for a
secondary mirror support.

[0022] In a preferred embodiment, the system of actuators has actuators
with translation axes perpendicular to the upper plane of the supporting
base. The system of actuators has six actuators distributed over the
periphery of the supporting base in order to displace the support of the
secondary mirror in six degrees of freedom. For this embodiment, the
mechanical structure bearing the support of the mirror preferably has six
mechanical arms, the length of which is equal to approximately one meter.

[0023] For the production of an autocorrected space telescope according to
the invention, the optical measurement means, the calculation means and
the system of actuators constitute an active control chain for correcting
the positioning of the secondary mirror in order to adjust the
observation configuration of the optical system.

[0024] Preferably, the calculation means compiles the positioning
corrections of the secondary mirror by means of a wavefront
reconstruction algorithm, and the system of actuators and the mechanical
structure bearing the support of the secondary mirror constitute
mechanical means intended to introduce defects on the measured images.
The on-board active control system determines the positioning corrections
to be provided at a given position on the basis of telescope image
measurements. The invention avoids the use of meteorological systems
coupled to the structure. This provides simplification of the systems for
the telescope, in cost and in mass.

[0025] The invention will be better understood, and other advantages will
become apparent, on reading the following description given nonlimitingly
and by virtue of the appended figures, in which:

[0026]FIG. 1 represents a simplified diagram of an existing solution for
an autocorrected space telescope having a hexapod for control of the
secondary mirror at the secondary mirror.

[0027]FIG. 2 represents a simplified diagram of a preferred embodiment of
the mechanical structure and the system of actuators for a hexapod of a
telescope having a primary mirror and a secondary mirror. For the sake of
clarity, the other elements of the telescope are not represented. The
secondary mirror is positioned in a first position in which the actuators
have the same configuration.

[0028]FIG. 3 represents a simplified structure of the same mechanical
structure and the system of actuators with the secondary mirror in a
second position. The actuators are controlled in order to be positioned
in different configurations. The representation of the displacement value
ranges of the actuators in the figure is also a simplified
representation.

[0029] FIG. 4 represents a simplified diagram of the same mechanical
structure. The mechanical structure is deployable and illustrates the
system in a position in which the secondary mirror rests directly on the
supporting base.

[0030] It is an object of the invention to make it possible to reduce the
mass of an optical system of the space telescope type and to improve the
agility of the optical system, in particular for an optical system having
a secondary mirror which may be deployable. The invention is not,
however, limited to optical systems with a deployable mechanical
structure. Specifically, one advantage of the invention is the design
flexibility of the optical system, the supporting base forming a standard
mechanical and control element on which the structure bearing the
secondary mirror is carried.

[0031] To this end, the invention as described by FIGS. 2 and 3 relates to
the mechanical structure of a high-resolution space telescope having a
primary mirror and a secondary mirror 4.

[0032]FIG. 2 represents a simplified diagram of the mechanical structure
of the space telescope with default positioning of the secondary mirror.
Each of the actuators controlling the positioning of the mirror is in the
same inactive position. The primary mirror is not represented for the
sake of clarity; it is positioned in the upper plane of the supporting
base 1. The support 3 of the secondary mirror 4 is carried by a
mechanical structure 2 on the supporting base 1, said supporting base 1
making it possible to control the positioning of the secondary mirror 4
by means of a system of actuators 5 executing a translation movement
perpendicular to the upper plane of the supporting base 1. The system of
actuators 5 has 6 actuators distributed over the periphery of the
supporting base. The movement is carried out on the lower end of each arm
21 to 26 of the mechanical structure.

[0033] During the operational phase of the space optical system when the
satellite is in orbit, said operational phase comprising the observation
phases and the phases of correcting the observation by modifying the
positioning of the secondary mirror, the length of the mechanical arms 21
to 26 is constant. The secondary mirror must be far enough away from the
primary mirror so that the focal plane of the images corresponds to the
detection plane of the image detection means of the optoelectronic
measurement means. Preferably, the length of the mechanical arms in the
operating configuration is about one meter. If the mechanical structure 2
is deployable, the length of said arms may be variable during the phase
of putting the optical system into operation. This phase generally takes
place after the satellite has separated from the launcher and been put
into orbit. If the satellite does not have a deployable secondary mirror
structure, the length of the arms is identical irrespective of the
operational phase. The choice of a deployment embodiment of the
mechanical structure 2 does not limit the scope of the invention.

[0034] The supporting base 1 also includes the system for active control
of the optics of the telescope. The figures do not represent the
electronic calculation and control means for the sake of clarity. An
optical measurement means, a calculation means and the system of
actuators 5 constitute an active control chain for correcting the
positioning of the mirror 4 in order to adjust the observation
configuration of the space telescope. The optoelectronic measurement
means generally consists of high-resolution electronic sensors, for
example of the CCD type (Charge-Coupled Device). These sensors are
positioned in the focal plane of the telescope. The calculation means
carries out image processing operations, on the basis of which data for
correcting the positioning of the secondary mirror 4 are compiled.

[0035] In another embodiment, the optical system includes the electronics
and the means for controlling the secondary mirror, while the calculation
means are located on the ground. The satellite carrying the optical
system then also has means of communication with the ground in order to
receive the correction data.

[0036] The image processing functions for calculating the positioning
corrections of the secondary mirror are preferably based on wavefront
reconstruction algorithms. By way of nonlimiting example, phase diversity
algorithms may be mentioned. The documents cited in the prior art
describe the methods of calculation by wavefront analysis. The phase
reconstruction consists in extracting the information about the optical
aberrations of the instrument, which are contained in the image, by using
numerical inversion methods. There are a plurality of methods and
hardware configurations for carrying out the calculations.

[0037] The principle of compiling corrections by wavefront analysis should
be recalled. This principle consists in evaluating positioning defects of
the secondary mirror via their impact on the image. On the basis of an
optical sensitivity matrix of the system, determined at the time of
designing the optical system, the effects of the misalignment of the
secondary mirror on the aberrations detected in the images are known. By
applying the inverse matrix to the images, the distance of which from the
focal plane is known, it is possible to recover an evaluation of the
misalignment of the secondary mirror and therefore an evaluation of the
corrections to be made.

[0038] In order to measure images having defects due to misalignment with
the focal plane, these defects are either introduced by additional
mechanical means, for example a means for displacing the image detector,
or by introducing an additional optical plate or by displacing the
secondary mirror. Preferably, the invention compiles the positioning
corrections of the secondary mirror by displacing the mirror to a known
position introducing defects on the recorded image. Nevertheless, the
method of introducing defects on the image in no way limits the scope and
spirit of the invention.

[0039] By the phase diversity method, the positioning of the secondary
mirror can be adjusted iteratively in order to approach an optimal
observation position. The method of measuring images and correcting the
positioning of the secondary mirror is carried out following the take-off
phase of the launcher, but also at multiple times in the mission so as to
ensure optimal performance of the telescope when confronted with ageing
phenomena of the structure and/or its materials.

[0040]FIG. 3 represents a simplified diagram of the optical system in a
configuration in which the mirror is misaligned so that an image detected
on the measurement means has defects. The positioning of the secondary
mirror is modified by translational movement of the actuators 51 to 56
located on the supporting base 1. Each actuator displaces the lower end
of a mechanical arm perpendicularly to the upper plane of the supporting
base 1. The displacement value range of the lower end of a mechanical arm
is approximately a few centimeters. The mechanical structure 2 comprises
mechanical arms 21 to 26, and each of the mechanical arms comprises a
pivot connection or a rotary connection at one of its ends and a rotary
connection at its other end, these rotary connections joining on the one
hand a mechanical arm to the support 3 of the secondary mirror and on the
other hand said mechanical arm to the actuator of the supporting base.
The connections may be formed in various ways: by using elements such as
universal joints, rolling bearings, bearing elements, but also flexible
elements or the flexibility of the arms themselves. The length of the
arms remains constant. This mechanical architecture thus makes it
possible to displace the orientation of the secondary mirror in 6 degrees
of freedom.

[0041] The calculation means compiling the correction data for positioning
the secondary mirror transmits these corrections to a control system of
the system of actuators 5. This control system converts the correction
data for positioning the secondary mirror into control data for each of
the actuators 51 to 56, said actuators carrying out altitude positioning
modifications of the bases of the mechanical arms. The purpose of the
control law of the actuators is to convert the instructions for
positioning the secondary mirror into altitude positioning of the
actuators 51 to 56.

[0042] The electronics and the mechanical means for controlling the optics
are positioned on a support base. The secondary structure 2 and 3 bearing
the secondary mirror 4 is preferably designed with mechanical elements
characterized by low passive dimensional stability requirements in
comparison with a mechanical architecture which is adjusted on the
ground, given that the optical adjustment is carried out in orbit. Once
in orbit, this structure does not experience strong mechanical stresses.
This latter structure is also more agile, the energy required for
modifying the configuration of said structure is also less and the
displacements are more precise. Overall, with a system of actuators
positioned on the supporting base, the optical system has a system for
autocorrection of the positioning of the secondary mirror which performs
better in precision and in correction efficiency.

[0043] FIG. 4 represents a mechanical structure bearing the support of the
deployable mirror such that the support of the secondary mirror rests
directly on the supporting base 1 in a first position. Thus, the space
telescope is installed inside a spacecraft and the mechanical structure
is configured in said first position when the system is installed in said
spacecraft. The deployable structure reduces the bulk of the optical
system and therefore makes it possible to transport a larger number of
systems inside the launcher.

[0044] The invention is intended in particular for space telescopes with
active control of the optics. The algorithms for controlling the
secondary mirror which have been described, merely by way of example, are
based on algorithms with wavefront reconstruction by phase diversity.
Nevertheless, the invention includes all variants which the person
skilled in the art may envisage without departing from the appended
claims.

[0045] The invention also preferably relates to telescopes with a
deployable secondary mirror, but is not limited to this type of
architecture.

Patent applications by Arnaud Liotard, Grasse FR

Patent applications by Frédéric Falzon, Pegomas FR

Patent applications by Guillaume Perrin, La Colle Sur Loup FR

Patent applications by Laurent Blanchard, Mouans-Sartoux FR

Patent applications by THALES

Patent applications in class Plural mirrors or reflecting surfaces

Patent applications in all subclasses Plural mirrors or reflecting surfaces